In traditional digital network transmissions, data bits are transported in fixed-bandwidth channels multiplexed into high-speed lines (e.g., fibers) in a Time-Division-Multiplex (TDM) manner, and the digital transmission hierarchy is organized into formats such DS-1 (1.5 Mb/s), DS-3 (45 Mb/s), or SONET (Synchronous Optical Network) formats, e.g., STS-x where x can range from 1 to 48 (50 Mb/s to 2.4 Gb/s). The SONET operation is also commonly known as the Synchronous Transfer Mode (STM), and thus the international standards (CCITT) version of the SONET as SDH (Synchronous Digital Hierarchy) where the signals are designated as STM-1 (150 Mb/s), STM-4 (622 Mb/s) and STM-16 (2.4 Gb/s).
Contrary to this concept of synchronous transmission of fixed-bandwidth channels, ATM (Asynchronous Transfer Mode) employs the notion of fixed-size cells (53-octet cells, octet=8-bit byte) to transmit arbitrary-bandwidth signals, and virtual channels can be set up between end points for variable rate transmissions. A key element to interchange between STM and ATM is the STM-to-ATM Converter (SAC).
The main function of an SAC is to take a fixed-bandwidth signal (e.g., DS-1, 1.5 Mb/s) and convert it into ATM cells for ATM transport. This process is called ATM adaptation (AAL in the standards). The main conversion process involves segmentation of the data into 48-octet payloads and the addition of a 5-octet header to each 48-octet piece to form 53-octet cells. It is customary to design a SAC for a fixed-bandwidth signal such as DS-1, DS-3, or STM-1. However, the payload of the STM-16 signal may be of different possibilities, e.g., four STM-4 (622 Mb/s), or 48 DS-3 signals, and so on. Constructing a specific SAC for each combination is cumbersome and expensive.
An object of the invention is to overcome the disadvantages of prior methods and systems.
Another object is to furnish a flexible device arrangement which can cope with a large variety of signal compositions and can perform ATM conversions on these different signal combinations.